# How much is (5x^6 + 9x^3 - 6x) + (-9x^7 - 20x^2 - 6x)?

$- 9 {x}^{7} + 5 {x}^{6} + 9 {x}^{3} - 20 {x}^{2} - 12 x$
Grouping by powers of $x$ this could be written as
$\textcolor{w h i t e}{\text{XXXX}}$$\left.\begin{matrix}\null & \null & + 5 {x}^{6} & + 9 {x}^{3} & \null & - 6 x \\ + & - 9 {x}^{7} & \null & \null & - 20 {x}^{2} & - 6 x\end{matrix}\right.$
$\textcolor{w h i t e}{\text{XXXX}}$$\text{-------------------------------------------------------}$
$\textcolor{w h i t e}{\text{XXXX}}$$\left.\left(\text{ } , - 9 {x}^{7} , + 5 {x}^{6} , + 9 {x}^{3} , - 20 {x}^{2} , - 12 x\right)\right.$